1. Field of the Invention
The present invention relates to an autofocus system, and more particularly to an autofocus system comprising an automatic tracking function that tracks a desired target subject as a focus object on a camera screen showing captured images.
2. Description of the Related Art
In imaging systems such as television cameras that convert images of a subject from an image pickup device (CCD or the like) into electrical signals (image signals), a contrast system is generally employed as an autofocus (hereunder, abbreviated as “AF”) system. A contrast system is a system which detects the contrast of a subject image from image signals that were captured by an image pickup device to automatically perform focusing to achieve the optimal focus state by controlling the focus of the taking lens such that the highest contrast is achieved.
In most AF employing a contrast system, the entire subject within the imaging region of the camera is not taken as the AF object, and instead only the subject within a region of one part of the imaging region is taken as the object. In AF according to a contrast system, image signals of the region taken as the AF object are extracted from the image signals incorporated from the entire imaging area of the image pickup device, and by controlling the focus such that the contrast of the extracted image signals of that region becomes the highest, the AF object is limited to only the subject of one part of the region. In the present specification, the region of a subject taken as the AF object among the imaging region of a camera or an image region of the subject taken as the AF object on a camera screen showing captured images is referred to as an “AF area”, and a frame showing the contours of that AF area is referred to as an “AF frame”.
Although, conventionally, when a single AF area is set after being fixed to a predetermined position on a screen showing captured images a rectangular AF area is normally set in the center of the screen, a device is also known that allows the position of the AF area to be changed by an operation of an operator (for example, see Japanese Patent Application Laid-Open No. 2002-365519).